1. Field of the Invention
The invention in general relates to amplifier devices, and more particularly to an FET (field effect transistor) which exhibits low noise.
2. Description of Related Art
In some electronic systems, a signal is produced indicative of some parameter to be measured. Generally the signal level is low and one or more amplifier stages are provided to increase the signal level. If noise is present in an amplifier the signal is corrupted.
For example, an FET amplifier device exhibits several types of noise such as white noise and 1/f noise. White noise is a noise which is constant across frequency, while 1/f noise is frequency dependent. That is, the lower the frequency, the larger the fluctuation of that noise. Such 1/f noise may be due to trapping whereby mobile carriers in the FET are trapped resulting in larger current fluctuations and a consequent increase in noise. Such trapping occurs mostly at defects at the interface of the FET semiconductor body and a surface oxide coating.
This 1/f noise is particularly bothersome in, for example, solid state imaging arrays which operate at low frequencies where the 1/f noise is becoming more pronounced. Therefore the sensitivity of such imaging arrays is severely degraded and desired viewing in extremely low light levels is degraded by noise. A bipolar transistor, by virtue of its bulk design has low levels of 1/f noise. These bipolar transistors however are not impedance compatible with certain sensors, and conventional FET transistors exhibit objectionable trapping and 1/f noise.
It is therefore an object of the present invention to provide an FET which ameliorates the objectionable drawbacks of prior art devices,